The Italian posted a 2m03.995s as the Pro drivers took to the track for the second time in the final 40 minute session of the day.
Supercars ace Chaz Mostert emerged second fastest overall after posting a 2m04.088s, driving the #65 Melbourne Performance Centre Audi, the time set in the opening practice session.
The Sun Energy1 Racing Mercedes outfit was third thanks to Luca Stolz’s effort late in Practice 4, while the #912 Manthey Porsche was fourth, 0.298s adrift of the pace.
Mostert topped the opening practice session as Friday's track action kicked off with the first of four 40-minute practice sessions, and one of just two open to the Pro drivers in the field.
The times tumbled quickly in the early part of the session, the first representative lap, a 2m05.235s, coming from Maro Engel in the #999 GruppeM Mercedes inside the first 10 minutes.
Fellow Mercedes driver Broc Feeney eclipsed that time in the #888 Triple Eight entry with a 2m05.136s, before former winner Matt Campbell became the first driver into the fours in the Manthey EMA Porsche with a 2m04.549s.
Just after the quarter hour mark the benchmark was lowered again, this time by Frederic Vervisch in the #55 Schumacher Audi who went quickest with a 2m04.350s.
That time proved tough to beat, the Fuchs-backed Audi holding on to top spot until the final two minutes.
By that point Supercars star Mostert was on a charge, steadily working his way up the order to end up on top with a 2m04.088s in the #65 Melbourne Performance Centre Audi.
That left Mostert leading Vervisch at the flag, with Tony D'Alberto a late improver as he put the #6 Wall Racing Lamborghini into third.
The Manthey Porsche ended up fourth thanks to Campbell's early lap, one spot ahead of the reigning winning entry, the #75 SunEnergy1 Mercedes.
The #99 Triple Eight Mercedes, the squad's Pro-Am entry, ended up sixth fastest as the #32 entry led the way for Team WRT BMW with the seventh fastest time.
MotoGP legend Valentino Rossi made a sensational debut on the Mount Panorama circuit as he topped the second session.
The session, limited to Bronze-ranked drivers, got off to an interrupted start, the red flag making its first appearance of the weekend a few minutes in when Keith Kassulke looped the #52 MARC V8 at The Cutting.
Once the session went green it was Brad Schumacher (#55 Schumacher Audi) and Kenny Habul (#75 SunEnergy1 Mercedes) that initially battled it out at the top.
That battle proved to be incredibly tight, Habul's 2m06.135s leading Schumacher's 2m06.139s with a little over 10 minutes to go.
It was at that point the pair were jumped by Rossi in the #46 WRT BMW, having been granted a dispensation for the session despite his Silver driver ranking.
Rossi was still leading when the race was red-flagged again inside the last 10 minutes after Marc Cini hit the wall at The Cutting in the #9 MPC Audi.
When the session resumed Bathurst local Schumacher made his play for top spot, jumping ahead of Rossi with a 2m05.518s right at the flag.
But Rossi was able to respond, the MotoGP legend reclaiming top spot with a 2m05.448s on his final run.
Kenny Habul dominated the third practice session in the #75 SunEnergy1 Racing Mercedes.
Reigning race winner Habul wasted little time getting down to the business in the second 40-minute session dedicated to Bronze-ranked drivers.
He effectively led the way from the word go in the #75 SunEnergy1 Mercedes, a series of improvements yielding a 2m05.309s in the early part of the session.
At the time that left Habul more than three seconds clear of the field, with Liam Talbot (#65 Melbourne Performance Centre Audi) and Schumacher (#55 Schumacher Audi) only closing the gap to around 1.5 seconds in the second half of the session.
They didn't get much closer, either, Talbot still more than nine-tenths adrift as he finished the session second fastest.
Schumacher was third, just over a second off Habul, after improving right at the flag.
The final Friday practice session for the Bathurst 12 Hour was cut short by a dramatic crash for MARC V8 driver Kassulke.
The veteran racer suffered a high-speed off at The Chase right on the midway point of the final 40-minute session of the day.
The car made contact with the tyre barrier on the exit of the corner before coming to a rest on its roof.
Early reports are that Kassulke was communicating with rescue crews before being extracted from the car.
That proved to be the end of the session due to damage to the tyre bundles.
That left Marciello and the #999 GruppeM Mercedes on top of the times as Friday came to a premature close.
Practice continues with a fifth session starting at 8:35am local time tomorrow.
Live coverage of the 2023 Bathurst 12 Hour will start on Motorsport.tv from Saturday onwards (not available to viewers in Australia and New Zealand).
